Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada J

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Atlantic Geology

Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada J. Gregory. McHone

Volume 47, 2011 Article abstract The island of Grand Manan (Canada) in the southwestern Bay of Fundy has the URI: https://id.erudit.org/iderudit/ageo47art06 only exposed strata and basalt of the Grand Manan Basin, a mainly submerged Early Mesozoic riﬅ basin about 30 km wide by 70 km long. The basin is See table of contents bounded on the southeast by the west-dipping Red Point Fault, which bisects the island, and on the northwest by a submarine border fault marked by the Murr Escarpment, a bathymetric feature that parallels the coast of Maine Publisher(s) (USA). A fault-bounded horst of Ediacaran to Cambrian rocks separates the Grand Manan Basin from the much larger Fundy Basin to the east. The Atlantic Geoscience Society Ashburton Head, Seven Days Work, and Southwest Head members of the end-Triassic Dark Harbour Basalt cover most of western Grand Manan with a ISSN total thickness around 240 m. Up to 12 m of sub-horizontal grey mudstone and fine-grained red sandstone of the Dwellys Cove Formation are exposed along 0843-5561 (print) the western shoreline beneath the basalt. Coarse red arkosic sandstone a few 1718-7885 (digital) metres thick at Miller Pond Road rests on a basement of Late Ediacaran rocks east of the basin. Exposures of the Dwellys Cove and Miller Pond Road Explore this journal formations are at the top and bottom, respectively, of several km (?) of sub-horizontal Late Triassic clastic basin strata, juxtaposed by the eastern border fault. Extending from beneath the Miller Pond Road sandstone, a level Cite this article surface of low relief forms much of the eastern island and its archipelago, which may be interpreted as a relict Late Triassic peneplain that was beneath McHone, J. G. (2011). Triassic Basin Stratigraphy at Grand Manan, New the Mesozoic strata. Brunswick, Canada. Atlantic Geology, 47, 125–137.

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J. Gregory McHone

9 Dexters Lane, Grand Manan, New Brunswick E5G3A6, Canada

Date received: 12 April 2011 ¶ Date accepted 26 June 2011

ABSTRACT

The island of Grand Manan (Canada) in the southwestern Bay of Fundy has the only exposed strata and basalt of the Grand Manan Basin, a mainly submerged Early Mesozoic rift basin about 30 km wide by 70 km long. The basin is bounded on the southeast by the west-dipping Red Point Fault, which bisects the island, and on the northwest by a submarine border fault marked by the Murr Escarpment, a bathymetric feature that parallels the coast of Maine (USA). A fault-bounded horst of Ediacaran to Cambrian rocks separates the Grand Manan Basin from the much larger Fundy Basin to the east. The Ashburton Head, Seven Days Work, and Southwest Head members of the end- Triassic Dark Harbour Basalt cover most of western Grand Manan with a total thickness around 240 m. Up to 12 m of sub-horizontal grey mudstone and fine-grained red sandstone of the Dwellys Cove Formation are exposed along the western shoreline beneath the basalt. Coarse red arkosic sandstone a few metres thick at Miller Pond Road rests on a basement of Late Ediacaran rocks east of the basin. Exposures of the Dwellys Cove and Miller Pond Road formations are at the top and bottom, respectively, of several km (?) of sub-horizontal Late Triassic clastic basin strata, juxtaposed by the eastern border fault. Extending from beneath the Miller Pond Road sandstone, a level surface of low relief forms much of the eastern island and its archipelago, which may be interpreted as a relict Late Triassic peneplain that was beneath the Mesozoic strata.

RÉSUMÉ

L’île Grand Manan (Canada) au sud‑ouest de la baie de Fundy possède la seule strate et basalte qui affleure du bassin de Grand Manan, un bassin d’effondrement en bonne partie submergé du début du Mésozoïque qui fait environ 30 km de large sur 70 km de long. Ce bassin est bordé au sud-est par la faille de Red Point qui présente en pend- age vers l’ouest et qui traverse l’île de part en part, et au nord‑ouest, par une faille de bordure sous‑marine marquée par l’escarpement Murr, une structure bathymétrique qui longe la côte du Maine (É.‑U.). Un horst bordé par une faille et composé de roches de la période comprise entre Ediacara et le Cambrien sépare le bassin de Grand Manan du bassin de Fundy, beaucoup plus vaste, à l’est. Les membres de Ashburton Head, de Seven Days Work et de Southwest Head de la formation de basalte de Dark Harbour, de la fin du Trias, recouvrent la plus grande partie de l’ouest de l’île Grand Manan, et leur épaisseur totale serait d’environ 240 m. Des roches de mudstone gris subhorizontal et de grès rouge à grains fins de la Formation de Dwellys Cove dont l’épaisseur peut atteindre 12 m affleurent le long du rivage occidental, sous le basalte. Une couche de grès arkosique rouge grossier de quelques mètres d’épaisseur sur Miller Pond Road repose sur un socle de roches de la fin d’Ediacara, à l’est du bassin. Des affleurements des Formations de Dwellys Cove et de Miller Pond Road sont observés respectivement au sommet et à la base de la strate subhorizontale de plusieurs kilomètres (?) d’un bassin de roches clastiques de la fin du Trias, en juxtaposition avec la faille de bordure orientale. Sous le grès de Miller Pond Road, une surface de niveau en terrain peu accidenté se prolonge et compose la plus grande portion de la partie est de l’île et de son archipel, ce qui pourrait suggérer la présence d’une ancienne pénéplaine du Trias tardif qui se trouvait sous la strate du Mésozoïque. [Traduit par la redaction]

INTRODUCTION known clastic and basalt units of the much larger Fundy Basin to the east, which is close enough to share a sedimentary and The Mesozoic section on Grand Manan has the only sur- structural history (Wade et al. 1996). face exposures of the Grand Manan Basin (Fig. 1), which is Unmetamorphosed basalt and clastic sedimentary strata on herein described as a separate Early Mesozoic rift basin, one of the island of Grand Manan were recognized as Early Mesozoic, many known along the eastern margin of North America (Ol- and similar to formations along the western coast of Nova Sco- sen 1997). The basin formations are comparable to the better- tia, in descriptions by Chapman (1870), Powers (1916), Alcock

Fig. 1. Digital bathymetry of the Grand Manan Basin area and topography of surrounding land areas. The coloured relief base map is by Roworth and Signell (1998), with structural borders in red after Wadeet al. (1996) and this paper. Line hachures indicate down-dip offset directions on normal faults. The darkest blue colour indicates where water depths exceed 125 m, with lighter colours nearer the surface. Shaded green colours simulate land relief.

(1948), Patrick (1964), Wade and Jansa (1994), and McHone the eastern border fault that coincides with the total thickness (2001). A generalized map by George Pajari and colleagues is of the sedimentary Triassic strata. shown in Allaby (1984, p. 42) and in a report for the Village of Grand Manan (Hart and Pajari 1988). The island’s current open-file bedrock map by Fyffe and Grant (2005) shows little THE GRAND MANAN BASIN detail for the Mesozoic formations and structures, but an up- dated version is in preparation (Fyffeet al. 2011). The Grand Manan Basin is a small Early Mesozoic rift basin This paper provides brief field descriptions of the Early Me- with maximum dimensions around 30 km wide by 70 km long sozoic formations of Grand Manan, with an overview of their (approximately 1300 km2), with its western margin about a km stratigraphic relationships and structural history. The juxta- offshore and parallel to the easternmost coast of Maine (Fig. 1). position of the top and bottom few metres of the sub-basalt The international boundary between Canada and the United sedimentary section on opposite sides of the island is a unique States extends through the central part of the basin, but the and unexpected occurrence, and it indicates displacement on border remains unresolved around Machias Seal Island a few

Fig. 2. Bedrock geology map of Grand Manan, simplified from Fyffeet al. (2011).

McHone – Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada Copyright © Atlantic Geology, 2011 atlantic geology . volume 47 . 2011 128 km to the south (Fig. 1). The western basin margin is well de- of the Bay of Fundy (Wade et al. 1996). Up to 8000 m of clas- fined by the Murr Escarpment, a straight and steep bathymetric tic sediments and 1000 m of basalt fill the central Fundy Ba- slope close to the Maine coast that apparently marks a high- sin, thinning to less than half that eastward into Nova Scotia angle Mesozoic border fault (Barnhardt et al. 1996; Dickson et (Fig. 3). In western Nova Scotia the Fundy strata generally dip al. 1994). The eastern basin border is located at the Red Point around 10°–15° to the northwest, due to normal faulting in the Fault on Grand Manan, which continues along a sharp break western part of the basin as well as subsidence toward south- in the bathymetric topography offshore to the south (Fig. 1). central areas (Wade et al. 1996). In contrast, the sedimentary The ends of the basin to the southwest and northeast are not strata and lower basalt flows on Grand Manan are generally accurately defined but are limited by bathymetric relief to the within 5° of horizontal except near faults, and with broad, gen- southwest, and by a seismic line across the Owen basin south tly dipping synclines and anticlines across the island (Fig. 2). of The Wolves islands that shows no Mesozoic strata (Fig. 2 of A large north-south basement fault block includes the Wade et al. 1996). Seismic survey lines reported by Tagg and eastern third of Grand Manan and its entire associated “ar- Uchupi (1964) and Dickson et al. (1994) crossed the south- chipelago” of smaller islands. This “White Head Island horst” western end of the Grand Manan Basin and indicate Mesozoic is bound on its eastern side by the submerged Grand Manan strata, but little about the basement structures. Fault, which lies within the Bay of Fundy and provides a south- Barnhardt et al. (1996) and Dickson et al. (1994) showed western border for the larger Fundy Basin (Fig. 1). The west- that the basin has a remarkably level and low-relief bathymetric ern border of the basement horst is the partially exposed Red depth around 70 to 90 m. The basalt flows and sedimentary Point Fault (Fig. 2), which is also the eastern border fault of strata on Grand Manan are generally within a few degrees of the Grand Manan Basin. horizontal, but with limited areas of wide gentle folds and a few Because the Mesozoic strata and basalt of the Grand Manan normal faults. The sub-horizontal strata are an exception to Basin are physically separate from formations of the Fundy the general description of Early Mesozoic rift basins in eastern Basin, new basin formation names are proposed (Fig. 2 and North America, which tend to be half-grabens with strata that following sections). Grand Manan metamorphic formations dip toward one major border fault (Olsen 1997). The indica- are also separated from similar formations elsewhere and have tion for the Grand Manan Basin is that its western and eastern been subject to revision with new work by Fyffe and Grant border faults have similar vertical displacements, and perhaps (2001, 2005), Black et al. (2004), and Miller et al. (2007). Re- moved simultaneously. lating tectonic terranes to the metamorphic basement rocks The most detailed thickness information for strata that of Grand Manan has been problematic, but field work and ra- might also represent the Grand Manan Basin is reported in diometric dates of 618 to 539 Ma suggest correlation with the a log for the Mobil Gulf Chinampas N-37 deep exploration New River and Mascarene terranes of New Brunswick (Miller well drilled east of The Wolves, within the western Fundy Ba- et al. 2007). Unlike the older rocks, the Mesozoic formations sin north of Grand Manan (Fig. 1). As reported in Wade et al. are directly comparable between the Grand Manan and Fundy (1996), the well encountered from top to bottom: 357 m of Mc- Basins (Fig. 3), and they may have been contiguous before tec- Coy Brook shale and siltstone; 333 m of North Mountain ba- tonic activity and erosion created the present map patterns. Dis- salt; 1157 m of Blomidon sandstone; and 1718 + m of Wolfville cussions of related “broad terrane” vs. “closed basin” models sandstone (not completely penetrated). The uppermost part of have been presented by McHone (1996), Wade et al. (1996), the basalt and any strata formerly above it on Grand Manan are Olsen (1997), Pe-Piper and Piper (1999), and McHone and not present or were eroded away. These formation thicknesses Puffer (2003), among others, and a related model for the Grand vary greatly across the Fundy Basin, and the Chinampas N-37 Manan Basin will be developed in further work on tectonic well can only be used for a rough estimate of formation thick- structures and basalts. nesses expected within the Grand Manan Basin. As described in more detail in the following sections, up to Machias Seal Island to the south of the Grand Manan Basin 12 m of mudstone and fine-grained sandstone of the herein- is a plutonic exposure of quartz monzodiorite, which yielded named Dwellys Cove Formation underlie the Dark Harbour an Early Cambrian U-Pb (zircon) age of 542.0 ± 0.9 Ma (Barr Basalt along much of the western shoreline, while a few metres et al. 2010). The adjacent mainland and other islands are com- of Triassic sandstone in a poorly defined area at Miller Pond prised of Paleozoic and Late Proterozoic metamorphic rocks of Road lie directly on metamorphic basement east of the Red the Ellsworth, Mascarene, New River, Brookville, and Kingston Point Fault (Fig. 2). The Dwellys Cove outcrops must be close terranes (Black et al. 2004) as well as Late Paleozoic granitic to the age of the basalt, but if the Miller Pond Road sandstone plutons (McLeod et al. 1994). exposures represent the base of the section, those rocks may be 15 to 20 million years older in the Norian age of the earliest The Fundy Basin basin sedimentation (Olsen 1997).

Late Triassic to Early Jurassic sedimentary and volcanic for- Dark Harbour Basalt mations of the Fundy Basin are well exposed along the western shorelines of Nova Scotia (Greenough 1995; Olsen 1997; Kon- Mesozoic exposures on western Grand Manan are domi- tak 2008) and can be studied from seismic surveys for much nated by the herein-named Dark Harbour Basalt. Although the

Fig. 3. Correlation and conceptual cross section sketch of Mesozoic formations of the Grand Manan Basin and Fundy Basin (ref. this paper; Wade et al. 1996). Strata thicknesses for Grand Manan are estimated from the Chinampas N-37 well log while Fundy Basin strata are maximum estimates, both from Wade et al. (1996). basalt characterizes much of Grand Manan, the name “Grand for the East Ferry member was discussed by Schaltegger et al. Manan” has already been used for a group of Late Neoprotero- (2008), and after several decades in which it was assigned to zoic formations on the eastern side (Fyffe and Grant 2001). the start of Early Jurassic, the North Mountain Basalt has been However, the basalt formation is well represented by easily ac- shown to be end-Triassic in age (Cirilli et al. 2009). The age as- cessed outcrops of its three members along the road to Dark signments invite a cause and effect relationship of this volcanic Harbour on the western coast of the island, and thus the name event with the great Triassic-Jurassic mass extinction (Deenen assignment. et al. 2010). The basalt has three members, which from top to bottom The Southwest Head Member at Southwest Head is about are: Ashburton Head Member (> 80 m), Seven Days Work 110 m thick and displays a colonnade or columnar structure Member (ca. 50 m) and Southwest Head Member (ca. 110 through its entire thickness, much like in a dyke or sill (e.g., Pali- m). Their petrology is similar to, and they are probably co- sades Sill) and without the expected entablature of contorted magmatic with, the Brier Island, Margaretsville, and East Ferry joints. About 10 to 15 m below the top of the Southwest Head members of the North Mountain Basalt in Nova Scotia (Fig. Member, a series of coarse segregation sheets or intrusional lay- 3) described by Kontak (2008). Additional descriptions of this ers of pegmatitic pyroxene and plagioclase crystals occurs near basalt were made by Greenough (1995), who pointed out that Whale Cove and Red Point. Philpotts et al. (1996) described this is one of the largest lava flows known, with an estimated the sheets as late-stage liquids squeezed upwards from a crystal 6,600 km3 present within the Fundy basin. The significance of mush of partially solidified basalt. The presence of segregation a U-Pb zircon date of 201.27 ± 0.06 Ma (Schoene et al. 2006)

McHone – Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada Copyright © Atlantic Geology, 2011 atlantic geology . volume 47 . 2011 130 sheets is a useful indicator of the location within the Southwest Head Member relative to its top and bottom. The lava lake represented by the Southwest Head Member was sandwiched between a base of relatively dry lakebed and a thick sequence of cooled lava flows of the Seven Days Work Member over it, so the basaltic liquid cooled slowly and uni- formly. A more detailed model for the emplacement of the basalt and its volcanic structures is a work in progress. A relatively level contact between the Southwest Head Mem- ber and underlying sedimentary Dwellys Cove Formation is exposed in a long section of the western shoreline (Fig. 2). Most measurements of bedding and the contact show very modest dip angles, with less than 5° west near Dark Harbour and a few degrees southeast near Dwellys Cove. A few small hills and valleys in the contact probably reflect original surface features of the silt and sand that were covered by the initial lava. Overlying the Southwest Head Member are approximately 12 to 14 vesicular lava flows, each a few metres thick, which Fig. 4. The Seven Days Work Member of the Dark Har- form the Seven Days Work Member (Fig. 4). The member is bour Basalt at the type locality northwest of Whale Cove. named after a nearly complete section that occurs along the Vegetation marks rubble tops or paleo-regoliths between shoreline referred to as Seven Days Work, northwest of Whale individual lava flows. The cliff face is about 50 m high. Cove. Small structures in this member include pinch and swell of flows, pahoehoe lava tongues, injections of narrow dykes and sills, erosional unconformities, rubble tops and glassy contacts, gaseous pipe vesicles, lava tubes and caves, and tumulus structures. The boundary between the top of the Southwest Head Mem- ber and overlying Seven Days Work lava flows is conformable but not horizontal where it is observed on the limbs of domes, synclines, and anticlines, which extend a few tens of metres to a kilometre or more wide (Fig. 2). The limbs of these structures dip from 10°–30° toward or away from the fold axes. This deformation is not observed along the contact with underlying sedimentary strata, which suggests that the lower Southwest Head Member was still mostly in a liquid state when the Seven Days Work Member lavas flowed over or onto it, forming a thick, layered crust on this giant lava lake within the original rift basin. Sags, domes, ridges, and other deformation features occur in these surficial lava layers, possibly from thickness varia- tions as well as from magmatic pressure and intrusions from Fig. 5. A high-angle fault under the vegetation in this cliff below. Additional instabilities resulted from density differences face at Indian Beach separating lower Southwest Head in the upper part of the lava lake caused by concentrations of Member (right side) from Seven Days Work (layered rising gas bubbles, which led to breakouts or dykes of vesicular flows, lower left) and upper Ashburton Head (upper left) basalt, and lava flows from those local sources. members of the Dark Harbour Basalt. The vertical offset A conformable contact between the Seven Days Work Mem- is approximately 120 m based on positions of the basalt ber and overlying Ashburton Head Member is gently dipping members. The steep joints are roughly parallel to the at its only observed location at Indian Beach (Fig. 5). The Ash- fault. The cliff face is about 80 m high, looking northeast. burton Head Member is interpreted to be a final and very thick lava flow, which is co-magmatic but caps the entire volcanic tively shallow, northeast-dipping normal fault, exposed on sequence in the basin. At the type locality of Ashburton Head beaches at either end, along which the Ashburton Head Mem- it is at least 60 m thick, with any higher levels and overlying ber was dropped down against the Seven Days Work Member. A formations probably removed by erosion. The basalt is colum- steeper normal fault trends east-southeast from Indian Beach, nar but commonly deformed. where it juxtaposes the Southwest Head Member basalt to the Several normal faults with significant displacements have south against Seven Days Work and Ashburton Head members been observed in shoreline cliffs, particularly in the northern to the north (Fig. 5). Interestingly, this fault is roughly co-lin- part of the island (Fig 2). Ashburton Head is bound by a rela- ear with the Oak Bay Fault on the mainland to the west, and

McHone – Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada Copyright © Atlantic Geology, 2011 atlantic geology . volume 47 . 2011 131 possibly with a fault near Pettes Cove on the eastern shore of Grand Manan (Fyffe and Grant 2001, 2005); however, inland exposures are too poor to trace it across the island. The Red Point Fault creates a major topographic and geological division on Grand Manan between Red Point and Whale Cove (Fig. 2), with a higher hilly and bog-rich upland underlain by Triassic basalt to the west, and a relatively low and level land- scape underlain by Ediacaran to Cambrian rocks to the east. A fault-line scarp marks its location through much of the island, but the fault is only exposed at Red Point (Fig. 6). Basalt of the Southwest Head Member (within 15 m of its top as indicated by segregation sheets) is drag-folded against red argillite or meta-volcaniclastic siltstone of the Cambrian (?) Long Pond Bay Formation (Fyffe and Grant 2005). The Red Point Fault dips about 50° to the west-northwest, and the shallow angle means that the map trace of the fault must bend west or east where it intersects lower and higher surface elevations. Basalt Fig. 6. The basin border fault at Red Point, looking north. columns that are nearly vertical about 20 m west of the fault Late Triassic columnar basalt of the Southwest Head become fractured and bent to a shallow angle as the fault is ap- Member of the Dark Harbour Basalt has moved down- proached, consistent with drag downward along the fault sur- ward on the left against Cambrian (?) argillite of the Long face (Fig. 6). Brecciated basalt is also present in several narrow Pond Bay formation on the right. zones along the cliff face to the west. As described in following sections, the vertical displacement along the fault apparently matches the total thickness of sedimentary strata beneath the basalt, which is estimated at ca.3 km. The low fault angle also requires displacement of the basin strata of 2–3 km westward, assuming mainly post-depositional tectonism.

Dwellys Cove Formation

This formation is named after Dwellys Cove on the southwestern shoreline of Grand Manan (Fig 2), where about 8 m of mudstone, siltstone, and fine-grained sandstone are exposed directly beneath the Southwest Head Member basalt. The formation occurs along about 14 km of the western shoreline, with the northern end at Money Cove Head where approximately 5 m is exposed, but the contact is not present beneath the basalt cliffs of the shorelines on the north side of the cove. The con- Fig. 7. Fine-grained maroon sandstone in a bank about tact and Dwellys Cove Formation are well exposed southward 4 m high underlies Cu-stained horizons of grey to buff along the shoreline, except in places of talus cover and for a siltstone at Dwellys Cove, near the “Sloop Cove Mine.” section about 1 km long south of Little Dark Harbour. The The contact with overlying Dark Harbour Basalt is mostly contact gradually lowers to below sea level near The Ladders covered by talus at this location. between Dwellys Cove and Sloop Cove (Fig. 2) and it has not been observed farther south. very little copper was probably found in 1870 and certainly in A locally famous mine adit was cut into siltstone of the a later exploration program by the Keevil Mining Group Ltd. Dwellys Cove Formation in a prospect for copper in 1870 (near (Patrick 1964). Part of one core from the Keevil program is on Fig. 7). A report by E. J. Chapman (1870) for the exploration display at the Grand Manan Museum in Grand Harbour, where group of his time describes the formation as mainly “buff col- it illustrates the major lithologies down to 660 feet (201 m). ored sandstone” but with 7 or 8 feet (ca. 2.5 m) of soft pale- The Keevil Mining Group exploration program for copper colored copper-bearing “tufa” directly beneath the basalt. The in the Dwellys Cove Formation (unnamed by the program) mine location was known as Sloop Cove, but more recent maps had a report by Patrick (1964) with this description on pp. locate Sloop Cove about a km farther south, where no sub- 4–5: “Underlying the basalt is a series of clastic sedimentary basalt strata are exposed. Because Dwellys Cove is close and rocks ranging from coarse sandstone to shale and mudstone has a good exposure of these strata, Dwellys Cove Formation with intermittent gypsum bearing horizons and veins, beds, is a more appropriate name. Although there are blue-green blebs and cavity fillings of acicular white sellenite. The sedi- stains and minor chalcocite close to the contact with the basalt, mentary rocks observed in outcrop consist of the top 30 to

40 feet of what is apparently a considerable thickness of Trias- pale grey mudstone shows cm-thin bands of blue Cu-mineral sic sediments underlying the Bay of Fundy. The deepest drill stains (azurite?), capping an off-white or bleached 40–80 cm hole gave a continuous section of some 380 feet of sediments thick zone that is harder (indurated), apparently as a hydro- which are mainly red sandstone with some greenish coloured thermal effect under the basalt (Fig. 9). Similar bleached sedi- members and red and green shale and mudstone near the top ment zones are noted beneath basalt elsewhere in the Fundy of the sequence. The upper most 20 feet of the sedimentary Basin and other rift basins, presumably due to reduction of sequence is cupriferous and was the main object of the explo- iron oxides in the clastic matrix. ration program.” The clay-rich mudstone is mainly confined to the uppermost Their comprehensive program of sampling, coring, and pet- 8 to 10 m of the Dwellys Cove Formation, and it is informally rochemical analyses showed only minor copper in thin zones referred to in this paper as the contact member. It might be pro- that were too low-grade to warrant further work. duced by a local playa or ephemeral lake in a semi-arid climate, Easier access to the Dwellys Cove Formation can be made as in the sedimentary environment described by Mertz and along the cobble beach north from Dark Harbour, where alter- Hubert (1990) for the Blomidon Formation. Wade and Jansa nating layers of pale grey siltstone and maroon clay-rich shaley (1994, p. 228) reported that this lithology is not known in the mudstone 10 to 50 cm thick (Fig. 8) appear to reflect Milankov- Blomidon Formation beneath the basalt elsewhere around the itch climate cycles as discussed by Olsen (1997). The mudstone Bay of Fundy. However, similar banded mudstone is present has numerous pits and spots with remnants of light-colored for a few meters beneath North Mountain Basalt at Partridge evaporite minerals, including gypsum. The contact with the Island in Nova Scotia (Olsen and Et-Touhami 2008), who in- basalt is about 7 m higher in this location, where a soft dusty clude a horizon in it that marks the end-Triassic mass extinction. More common in the Blomidon Formation is “redbed” sandstone (Mertz and Hubert 1990). This appears to be true for the Dwellys Cove Formation as well, and light red, fine- grained sandstone makes up most of the clastic part of the core on display in the Grand Manan Museum, which is missing the clay-rich contact member directly beneath the basalt. Core logs for 9 wells in the program reported by Patrick (1964) have a maximum well depth through 181 ft (55.1 m) of arenaceous “brick-red” to mottled-colour sandstone beneath the contact member, which has 15–35 ft (4.6–10.7 m) of grey to red (maroon) argillaceous siltstone and shale. The author found an exposure of similar red, fine-grained arenaceous sandstone in a near-shore pavement about 1 km north of Dark Harbour (Fig. 10), where it occurs about 12 m

Fig. 8. Alternating layers of siltstone and clay-rich mudstone of the Dwellys Cove Formation north of Dark Harbour. The scale bar is 6 inches/15 cm.

Fig. 10. Evaporite mineral patches and eroded vugs in the bedding surface of fine-grained, brick-red sandstone of the Dwellys Cove Formation north of Dark Harbour. The coin Fig. 9. Bleached zone beneath basalt north of Dark has a diameter of 23 mm. The end of a hammer appears at Harbour. Note the hammer, lower left. the bottom of the photo.

McHone – Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada Copyright © Atlantic Geology, 2011 atlantic geology . volume 47 . 2011 133 beneath the contact with the basalt. The bedding surface dips about 5° westward (seaward), which might be the reason the overlying basalt is relatively unstable, as its columns are per- pendicular to the bedding and thus tilt outward from the cliff face. Patches and holes in the bedding surface are related to evaporite minerals (Fig. 10), similar to mineral patches in the mudstone of the contact member. An unidentified green mineral occurs with the gypsum (Fig. 10). Minerals that have been eroded to form the open spaces are also unidentified but may be described in other studies of the Blomidon Formation (Mertz and Hubert 1990; Olsen and Et-Touhami 2008).

Miller Pond Road Formation

Alcock (1948) noted the presence of “minor amounts of red- dish and brownish sandstone and conglomerate” a few km to the southwest of Grand Harbour, which he mapped beneath Fig. 11. Feldspar-bearing red arenaceous sandstone with the basalt along the western side of the Red Point Fault. Alcock scattered clasts of grey-green phyllite from an abandoned apparently did not see the distinctly different Dwellys Cove gravel quarry near Miller Pond Road. The coin has a siltstone beneath basalt, as his map shows none along the west- diameter of 21 mm. ern shoreline. Wade et al. (1996 p. 213) mentioned that they looked for, but did not find Alcock’s eastern sandstone. The map by Fyffe and Grant (2005) reproduces the same outcrop area shown by Alcock (1948). This sandstone was relocated by the author in the spring of 2010 (Fig. 11), exposed partly under brush along a bank on the southeastern side of an abandoned gravel quarry off Miller Pond Road, and within the area where it was originally mapped (Fig. 2). The purplish red quartz arenite is medium to coarse grained, tending to arkosic with up to 10% feldspar grains, a trace of clay in the matrix, and weakly cemented by iron oxide and calcite. Bedding is difficult to discern but appears to dip about 5–10° toward the northeast. The sandstone at the northern end of the bank lacks foreign rock fragments, but a few meters toward the south it contains scattered clasts (Fig. 11) of grey-green to rusty phyllitic quartzite or meta-volcaniclastic siltstone similar to the major lithology of the Ingalls Head Formation to the east Fig. 12. Unconsolidated clast-supported phyllite breccia (Fig. 2). The clasts are angular and range around 0.2–4.0 cm of the Ingalls Head Formation grades toward the upper in length. This phyllite appears to be the only clast lithology right into pebbly red sandstone of the Miller Pond Road present in the Miller Pond Road sandstone. Formation. The diameter of the coin is 23 mm. Outcrops in the bank about 10 m to the southwest are down- dip, and thus a meter or so stratigraphically below the level mity at this location is obscured by vegetation and the inter- of the conglomeratic sandstone outcrop. Here the sandstone mixing of the formations in a paleo-regolith. grades into a mass of angular pieces of the same phyllite, be- Columnar basalt of the Southwest Head Member crops out coming the matrix of clast-supported breccia or paleo-regolith in the hillside about 70 m to the northwest, just outside the level (Fig. 12). The angular fragments preserve a fabric or preferred grounds of the gravel quarry. The Red Point Fault apparently orientation that might represent original cleavage or foliation is located near the base of the hillside, requiring the sandstone in the bedrock, which appears to be dipping toward the north to be east of the fault and not beneath the basalt as shown by more steeply than the sandstone bedding. Alcock (1948) and Fyffe and Grant (2005). The sandstone does The field evidence indicates that the sandstone was depos- not resemble a basin margin alluvial conglomerate or fanglom- ited upon and grades into the top of the Ingalls Head Forma- erate, which would typically contain a mix of rounded clasts of tion, although solid bedrock of this basement rock is not locally durable lithologies derived from adjacent highlands. This ex- exposed. Surface exposures of some metamorphic basement posure of Miller Pond Road sandstone is interpreted to be the rocks are observed to be brecciated into present-day angular bottom of the Triassic stratigraphic section beneath the basalt, gravel at other locations, apparently a function of rock cleavage not originally adjacent to the basalt or fault-bounded basement and fracture patterns. The exact appearance of the unconfor- rocks, and it is preserved on the White Head Island horst as a

McHone – Triassic Basin Stratigraphy at Grand Manan, New Brunswick, Canada Copyright © Atlantic Geology, 2011 atlantic geology . volume 47 . 2011 134 remnant equivalent to the base of the Wolfville Formation of the Fundy Basin (Fig. 3). Because the top of the island’s sedimentary section is exposed at only a slightly lower elevation along the shoreline to the west, and the strata are relatively horizontal, the vertical offset of the basin border fault must coincide by remarkable chance with the total thickness of the Triassic sedimentary section. As discussed earlier, sub-basalt strata totals around 2850 m in the Mobil Gulf Chinampas N-37 exploration well to the north of Grand Manan (Wade et al. 1996), which provides an estimate for the Grand Manan Triassic section and border fault offset.

Topographic Features

East of the Red Point Fault, the metamorphic basement Fig. 13. View southward at low tide from the harbour at rocks of Grand Manan have a generally low and level surface Woodwards Cove, across mainly meta-volcaniclastic ledges of less than 30 m elevation (except north of the Pettes Cove of Ingalls Head, Thoroughfare, and Priest Cove forma- Fault), with shallow harbours and low islands off shore (Fig. tions. Similar low elevations and small relief characterize 13). This gentle topography has been much more suitable for most of the island east of the border fault. small-boat harbours, farms, and residential areas than the hilly uplands of Grand Manan west of the fault, where there are many bogs and steep basalt ledges. The contrasting topography is shown on a digital relief map (Fig. 14), and it is not consistent with differential erosion that might be expected to develop from the variety of lithologies and structures of eastern Grand Manan. Coastal areas of the adjacent mainlands of Maine and New Brunswick are generally more rugged, with 10–40 m headlands of wave-cut cliffs and drowned valleys (Dickson et al. 1994; Wade et al. 1996, p. 197). Those areas are west of the Grand Manan and Fundy rift basins, whereas eastern Grand Manan is an exposed part of the White Head Island basement horst between the basins (Figs. 1, 2, and 3).

DISCUSSION

An explanation of why the eastern topography is so level is provided by the presence of the Triassic sediment at Miller Pond Road. This sediment is interpreted to be the base of the strata that eventually filled the Grand Manan and Fundy rift basins, starting around 215 Ma (Olsen 1997). Because it is only weakly cemented, the sandstone must have poor resistance to erosion, and the area of the sandstone is not lower or much better protected than other areas of the eastern island. It is proposed that the Miller Pond Road sandstone exposure is the last remnant of several km of Triassic basin formations that originally covered the present level metamorphic surface of eastern Grand Manan, possibly up to relatively recent times. In that case, much of the modern topography of eastern Grand Manan was developed during the pre-Late Triassic Pe- riod, and it now represents a paleo-peneplain, or the same sur- Fig. 14. Digital relief map of Grand Manan, showing the face upon which the Mesozoic basin sediments first started to contrast in relief and topography on either side of the collect around 215 Ma. In this model, normal faulting created border fault. Base map source: Service New Brunswick the horst in the Jurassic Period, and regional uplift since the (2010).

Cretaceous (Roden-Tice and Wintsch 2002) led to erosion of encourage the interpretation of the Miller Pond Road as pre- the overlying strata. This ancient interface surface and Grand sented herein, but better comparative studies would be useful. Manan Basin strata have remained relatively level since then, despite major tectonic activity that has tilted Mesozoic strata in other areas around the Bay of Fundy. SUMMARY The model developed for the Grand Manan Basin includes the truncation of strata and basalt by post-deposition faulting, 1. The Grand Manan Basin is a small Early Mesozoic rift ba- rather than thinning and on-lap with alluvial conglomerates sin in southern-most New Brunswick and eastern-most Maine, against borders that were tectonically active before and during which is structurally distinct from the Fundy Basin but with deposition. This interpretation conflicts with that of Wade et al. similar Triassic formations. Overlying Jurassic strata are not (1996) for the subsurface adjacent to the Grand Manan Fault on present. the eastern side of the White Head Island basement block (Fig. 2. The eastern border of the basin bisects the island of Grand 1), in which basin strata become tilted and thinner against the Manan in a north-south direction and is exposed as a west- fault border. The Grand Manan Basin must also move equally dipping normal fault at Red Point. The western border is sub- downward along its western border for the strata and basalt to merged near the coast of Maine, and it is interpreted as a fault maintain their original sub-horizontal orientations and thick- with a similar displacement so that strata in the basin remain nesses, as mapped across the island up to the Red Point bor- generally horizontal. der fault. The model by Wade et al. (1996) is consistent with a 3. Dark Harbour Basalt covers most of the island and is closed-basin interpretation, whereas the Grand Manan Basin comprised from top to bottom of the Ashburton Head, Seven observations indicate the former continuity of Triassic strata Days Work, and Southwest Head members. The uppermost and basalts across areas between basins, followed by faulting few meters of Dwellys Cove grey siltstone and red sandstone and separation. Roden-Tice and Wintsch (2002) presented a are exposed beneath and in contact with the basalt along the similar model for the Hartford Basin and surrounding areas of western shoreline. southern Connecticut (USA), based on fission-track ages for 4. A few meters of Late Triassic (?) Miller Pond Road red Late Jurassic and Cretaceous uplift and tectonic tilting. sandstone rest upon and merge into paleo-regolith of Late Neo- The two models might agree better for post-basalt features, proterozoic (Ediacaran) phyllite of the Ingalls Head Formation. in which alluvial breccias and basement-clast conglomerates 5. With the interpretation of uppermost and lowermost Tri- were developed in Jurassic strata along some border faults ac- assic sub-basalt strata exposed west and east of the Red Point cording to tectonic action soon after the volcanism (McHone Fault, the fault displacement must equal the thickness of the and Puffer 2003). This process is indicated by paleo-talus along sedimentary section of the basin. The best available estimate the northern end of the Fundy Basin (Olsen and Et-Touhami for this thickness is about 2950 m, as found in an exploration 2008), by regional tilting of all basin strata toward border faults, well in the Fundy Basin nearby to the north. and by Jurassic border fanglomerates mapped in rift basins in eastern North America (Olsen 1997). Such features are missing at Grand Manan. ACKNOWLEDGEMENTS Another question is the assignment of the Miller Pond Road sandstone outcrop to the base of the Triassic stratigraphic sec- Nancy McHone provided field assistance and much other tion. Post-metamorphic sandstone interpreted to be Late Trias- help during numerous years for this project. The author ap- sic is present in the Lepreau Formation on the coast of the New preciates field visits or discussions of Grand Manan geology Brunswick mainland to the north of Grand Manan, but the na- with Sandra Barr, Jelle de Boer, Richard Grant, Leslie Fyffe, ture of its contacts with surrounding metamorphic rocks of the Dan Kontak, Paul Olsen, George Pajari, Anthony Philpotts, Brookville terrane and its correlation with other Triassic rocks John Puffer, Peter Thompson, and Chris White. John Hubert is not clear (McLeod et al. 1994; MacNaughton and Pickerill and an anonymous reviewer made many helpful suggestions 2010). Olsen and Et-Touhami (2008, p. 10) described Permian for the manuscript. fluvial sandstone formations between the Late Triassic strata and older basement rocks in the northern Fundy Basin, and they suggest that the Lepreau Formation is also of that age. If REFERENCES so, perhaps the Miller Brook Road sandstone is Permian as well. 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